Previously, antisera against recombinant outer capsid rotavirus VP4 proteins were used to study the VP4 antigenic polymorphism among human rotavirus strains. Three distinct serotypes and one subtype of the VP4 protein were identified among 17 human rotavirus strains that contained representatives of five different VP7 serotypes. Results from cross- immunoprecipitation and neutralization assays with VP8 antisera suggest that the VP8 cleavage subunit of VP4 contains the major antigenic sites responsible for the serotype-specific neutralization of rotavirus via VP4. Immunization of mice with the VP8 cleavage subunit of VP4 of the human rotavirus KU, DS-1, 1076, or K8 strain induced polyclonal antibodies that neutralized the rotavirus from which the VP4 subunit was derived. However, most of the VP4-monoclonal antibodies obtained so far have recognized heterologous strains. We will attempt to produce serotype-specific monoclonal antibodies (MAbs) against VP4 using "mosaic virus" produced by coinfection of insect cells with baculovirus recombinant expressing VP6, VP7, and VP4 genes.